Tungsten plug corrosion prevention method using water

ABSTRACT

Disclosed herein is a method of making integrated circuits. In one embodiment the method includes forming tungsten plugs in the integrated circuit and forming electrically conductive interconnect lines in the integrated circuit after formation of the tungsten plugs. At least one tungsten plug is electrically connected to at least one electrically conductive interconnect line. Thereafter at least one electrically conductive interconnect line is contacted with water for a period of time less than 120 minutes.

BACKGROUND OF THE INVENTION

Interconnect lines electrically connect devices within an integratedcircuit (IC). IC devices may include one or more complimentary metaloxide semiconductor (CMOS) transistors having diffused source and drainregions separated by channel regions, and gates that are located overthe channel regions. In practice, an IC may include thousands ormillions of devices, such as CMOS transistors.

Interconnect lines of ICs generally take the form of patternedmetallization layers. Interconnect lines may be formed one on top ofanother with an electrically insulating material therebetween. As willbe more fully described below, one interconnect line may be formed underanother interconnect line and electrically connected thereto by one ormore tungsten plugs.

ICs are manufactured on silicon substrates using conventionalphotolithographic techniques. FIGS. 1-8 show a cross-sectional view ofan IC during a portion of its manufacture. More particularly, FIG. 1shows a first dielectric layer 12, a first metallization layer 14, and aphotoresist layer 16 formed over substrate 10. Layers 12-16 are formedusing conventional techniques such as chemical vapor deposition,sputtering, or spin-on coating.

First metallization layer 14 can be formed into a first interconnectline. This first interconnect line can be formed by selectively exposingphotoresist layer 16 to light passing through a patterned reticle (notshown). Photoresist areas of layer 16 exposed to light are subsequentlyremoved using conventional development techniques. FIG. 2 shows thesubstrate 10 of FIG. 1 after development of photoresist layer 16 to formphotoresist mask pattern 20.

Once the photoresist mask pattern 20 is formed, a plasma etchingoperation is applied to the IC shown in FIG. 2 to remove portions ofmetallization layer 14 that are not covered by photoresist mask pattern20. FIG. 3 shows the IC of FIG. 2 after plasma etching thereof. Theplasma etching operation results in first interconnect line 22.

FIG. 4 shows the IC of FIG. 3 after a second dielectric layer 24 isdeposited thereon. Although not shown, photoresist mask pattern 20 isremoved prior to formation of second dielectric layer 24. The seconddielectric layer 24 and the first dielectric layer 12 may be formed froman insulating material such as silicon dioxide.

FIG. 5 shows the IC of FIG. 4 after a via 26 is formed within the seconddielectric layer 24. As is well known in the art, vias, such as via 26,are formed by depositing a photoresist layer (not shown) over dielectriclayer 24, selectively exposing this photoresist layer to light passingthrough a patterned reticle having via hole patterns formed therein,developing and removing the exposed photoresist to form a photoresistvia mask pattern, etching any dielectric layer 24 exposed through thephotoresist via mask pattern, and removing the remaining photoresist viamask after etching dielectric layer 24.

Once the vias are formed within the second dielectric layer 24, the viasare filled with an electrically conductive material such as tungsten. Aswell is known in the art, vias, such as via 26, are filled by depositinga barrier film by sputter or chemical vapor deposition, depositing aconductive film by sputter or chemical vapor deposition, and thenremoving the conductive film, and possibly removing the barrier film,over dielectric layer 24, but not inside the via 26. The barrier film istypically comprised of titanium, titanium nitride, or atitanium/titanium nitride stack. The conductive film is typicallytungsten. The conductive film, and possibly the barrier film, is removedby plasma etching, chemical mechanical polishing, or wet etching. FIG. 6shows via 26 of FIG. 5 filled with tungsten, thereby forming tungstenplug 30.

After the tungsten plugs are formed, a second metallization layer isformed over dielectric layer 24 and the tungsten plugs, includingtungsten plug 30. This metallization layer is typically comprised of ametal stack that includes any combination of one or more the following:titanium, titanium nitride, aluminum, an aluminum copper alloy, or analuminum silicon copper alloy. This metallization layer is thenpatterned using conventional photolithography and plasma etching to forman additional layer of interconnect lines. FIG. 7 shows the IC of FIG. 6with a second interconnect line 32 formed thereon. The secondinterconnect line 32 is electrically coupled to the first interconnectline 22 via the tungsten plug 30. First interconnect line 22 may becoupled at one end to a first device (i.e., a first CMOS transistor).The second interconnect line 32 may be coupled to a second device (i.e.,a second CMOS transistor) or coupled to connections which lead to theoutside of the chip package. Accordingly, the structure of the firstinterconnect line 22, tungsten plug 30, and second interconnect line 32,function to interconnect the first and second IC devices or function tointerconnect an IC device and external package connections.

As is well known in the art, conventional plasma etching to forminterconnect lines (e.g., interconnect line 32) often leaves residualpolymer (not shown) on the sides of the interconnect lines. To removethis residual polymer on the sides of the interconnect lines, a liquidcleaning solution is often used after plasma etch. Further, conventionalplasma etching to form interconnect line 32 may leave a positiveelectrical charge interconnect line 32, and thus, tungsten plug 30 andfirst interconnect line 22. For purposes of explanation, it will bepresumed that the structure consisting of first interconnect line 22,tungsten plug 30, and second interconnect line 32 is a floatingstructure such that both interconnect lines 22 and 30 and tungsten plug30 will be positively charged before the polymer residue removalprocess.

After plasma etching, the IC shown in FIG. 7 is exposed to a cleaningsolution to remove any polymer remaining after the plasma etching step.Typically this cleaning solution may be alkaline or basic in nature(i.e. pH is greater than 7), however, acidic solutions (i.e. pH is lessthan 7) can also be used. Although the cleaning solution works well inremoving polymer residues, one, some, or all of the tungsten plugs thatare exposed to the cleaning solution may dissolve or erode away duringthe polymer residue removal process. The cause is electrochemicalcorrosion caused by two dissimilar conductive materials being incontact, the interconnect line and the tungsten plug, while bothconductive materials are simultaneously in contact with an electrolyte,the cleaning solution or rinsing solution, during the polymer removalprocess.

More and more devices are packed into smaller ICs. As such, the densityof devices and interconnect lines in ICs has dramatically increased overthe years. Unfortunately, this dense integration of devices andinterconnect lines has the effect of pushing the limits of conventionalphotolithography patterning, which necessarily makes photolithographymasks misalignments more likely to occur. An increase in misalignmentswill result in an increase of exposed tungsten plugs.

FIG. 7 illustrates the effects of misalignment of photolithographymasks. More particularly, the misalignment of photolithography masksused to create second interconnect line 32 produces a misalignment ofsecond interconnect line 32 with respect to tungsten plug 30. As aresult of this misalignment, tungsten plug 30 will be exposed tocleaning solution during the polymer residue removal step describedabove.

FIG. 8 illustrates how tungsten plug 30 could be corroded by thecleaning solution of the polymer residue removal process. As seen inFIG. 8, a substantial portion of tungsten plug 30, is removed by theaforementioned corrosion. Tungsten plug corrosion may have adverseeffects on performance of the IC. For example, corrosion of tungstenplug 30 shown in FIG. 8 may be so extensive that first interconnect line22 is no longer electrically coupled to second interconnect line 32thereby creating an open circuit therebetween. IC devices coupled tosecond interconnect line 32 could be electrically isolated from ICdevices coupled to first interconnect line 22 thereby resulting in an ICthat fails to function for its intended purpose.

Clearly, there is a need to avoid tungsten plug corrosion in themanufacture of ICs. In 1998, a paper was published by S. Bothra, H. Sur,and V. Liang, entitled, “A New Failure Mechanism by Corrosion ofTungsten in a Tungsten Plug Process,” IEEE Annual InternationalReliability Physics Symposium, pages 150-156. This paper, which isincorporated herein by reference in its entirety, describes sometechniques for preventing tungsten plug corrosion. These techniquesinvolve discharging the tungsten plugs prior to immersion in alkalinecleaning solution to remove polymer residue. In one technique describedin the paper, tungsten plug discharge is accomplished by dipping ICs inan ionic solution prior to polymer residue removal. The paper describesthat this ionic solution should have a pH near neutral (e.g. pH near 7).The paper describes deionized (DI) water as one form of ionic solutionfor discharging tungsten plugs. However, the paper found that arelatively long emersion time of several hours within the DI water wasnecessary to discharge exposed tungsten plugs, such as the exposedtungsten plug shown in FIG. 7. The exposed tungsten plugs were found toremain in tact after subsequent emersion in the alkaline cleaningsolution; however, noticeable corrosion of the interconnect lines, suchas interconnect line 32, was observed. Accordingly, this paper concludedthat emersion in DI water of ICs for the purpose of discharging exposedtungsten plugs, was not a “practical” approach. It is noted that thispaper should not be considered prior art to the invention claimedherein.

U.S. Pat. No. 6,277,742 describes another technique for preventingtungsten plug corrosion. In U.S. Pat. No. 6,277,742, an IC is dippedinto an electrolyte solution sufficiently acid or alkaline. According toU.S. Pat. No. 6,277,742, charges accumulated can be discharged bydipping the IC into the electrolyte solution. Preferably, when an acidelectrolyte solution is used, the pH value of the acid electrolytesolution is said to be less than 6.5. The acid electrolyte solution issaid to include an oxy-acid aqueous solution such as acetic acid(CH₃COOH), sulfuric acid (H₂ SO₄) or nitric acid (HNO₃). The acidelectrolytic solution is said to include a hydrohalic acid likehydrofluoric acid (HF) or hydrochloric acid (HCl). An acid salt aqueoussolution, for example, sodium hydrogen sulfate (NaHSO₄), ammoniumchloride (NH₄Cl) or ammonium nitride (NH₄NO₃) is also said to besuitable. Preferably, when an alkaline electrolyte solution is used, thepH value of the alkaline electrolyte solution is greater than 7.5. Thealkaline electrolyte solution is said to include either ammoniumhydroxide (NH₄OH) aqueous solution or metal hydroxide (M(OH)_(x))aqueous solution. The metal hydroxide aqueous solution includes sodiumhydroxide (NaOH) or potassium hydroxide (KOH). An alkaline salt aqueoussolution, for example, sodium acetate (CH₃COONa) or sodium carbonate(Na₂ CO₃) is also said to be suitable. It is noted alkaline or acidicelectrolytic solution may be environmentally hazardous or hazardous tothose who are responsible for discharging ICs prior to polymer residueremoval.

SUMMARY OF THE INVENTION

Disclosed herein is a method of making integrated circuits. In oneembodiment the method includes forming tungsten plugs in the integratedcircuit and forming electrically conductive interconnect lines in theintegrated circuit after formation of the tungsten plugs. At least onetungsten plug is electrically connected to at least one electricallyconductive interconnect line. Thereafter at least one electricallyconductive interconnect line is contacted with water for a period oftime less than 120 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood in its numerous objects,features, and advantages made apparent to those skilled in the art byreferencing the accompanying drawings.

FIG. 1 is a cross-sectional view of a portion of a partially fabricatedintegrated circuit;

FIG. 2 shows the IC of FIG. 1 after patterning the photoresist layer toform photoresist mask pattern;

FIG. 3 shows the IC of FIG. 2 after etching the first metallizationlayer;

FIG. 4 illustrates the IC of FIG. 3 with a second dielectric layerformed thereon;

FIG. 5 illustrates the IC of FIG. 4 after formation of a via within thesecond dielectric layer;

FIG. 6 shows the IC of FIG. 5 with a tungsten plug formed therein;

FIG. 7 shows the IC of FIG. 6 after formation of a second interconnectline thereon;

FIG. 8 shows the IC of FIG. 7 after exposure to a cleaning solution toremove polymer residue;

FIG. 9 is a graph showing test results of an IC manufactured with andwithout use of one embodiment of the present invention to dischargetungsten plugs; and

FIG. 10 is another graph showing test results of an IC manufactured withand without use of one embodiment of the present invention to dischargetungsten plugs.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

The present invention relates to a method of making ICs. In oneembodiment the method includes forming a tungsten plug in a dielectriclayer and forming an electrically conductive interconnect line partiallyor completely covering the tungsten plug after formation of the tungstenplug. FIG. 7 illustrates an exemplary, partially formed IC in whichinterconnect line 32 is formed after formation of dielectric layer 24and tungsten plug 30. The electrically conductive interconnect line 32in FIG. 7, may be formed from conductive materials such as a metal stackcomprised of any combination of one or more of the following: titanium,titanium nitride, aluminum, an aluminum copper alloy, or an aluminumsilicon copper alloy. The Tungsten plug 30 is electrically connected toconductive interconnect line 32.

Formation of conductive line 32 may result in an unwanted polymerresidue as described above. Moreover, formation of conductive line 32may result in the accumulation of electrical charge on the conductiveline 32 and the tungsten plug 30 connected thereto and the underlyingconductive line 22 connected to tungsten plug 30. The polymer residuemay be removed by a step of exposing the partially formed IC of FIG. 7to a cleaning solution. Before the polymer residue removal step, butafter the formation of the conductive interconnect line 32, thepartially formed IC is brought in contact with water for a period oftime less than 120 minutes. More particularly interconnect line 32connected to tungsten plug 30 (and tungsten plug 30 if not covered byinterconnect 32), is contacted with water for a period of time less than120 minutes. In one embodiment, contact is effected by dipping thepartially formed IC into a bath of water. In another embodiment, thewater is sprayed on the IC. In another embodiment, the water isdispensed on the IC by a nozzle. In a preferred embodiment, interconnectline 32 is contacted with water for a period of time equal to or lessthan 15 minutes.

The contact with the water fully or partially discharges conductiveinterconnect line 32 and tungsten plug 30 connected thereto and theunderlying conductive line 22 connected to tungsten plug 30. It is notedthat ICs may be created with more than two levels of interconnect lines.Interconnect lines 22 and 32 in FIG. 7 are lines in two separate levels.Ideally, each time a level of interconnect lines is formed, the newlyformed interconnect lines should be contacted with water.

The water used to discharge conductive interconnect line 32 and/ortungsten plug 30 connected thereto and/or the underlying conductive line22 connected to tungsten plug 30, may have a pH at neutral or 7. It isnoted that the pH of the water may be slightly higher or lower thanneutral. In one embodiment, the water used is degasified. Degasifiedwater can be formed during a distillation and/or filtration process inwhich as much of the dissolved gases (i.e., nitrogen, oxygen, carbondioxide, etc.) and microbubbles as possible are removed from water. Inanother embodiment, water that is not degasified and which has a pH ator near neutral, is used to discharge conductive interconnect line 32and tungsten plug 30 connected thereto and the underlying conductiveline 22 connected to tungsten plug 30. The water used to dischargeconductive interconnect line 32 and tungsten plug 30 connected theretoand the underlying conductive line 22 connected to tungsten plug 30 maybe deionized (DI) water. DI water is water which has been “deionized” orhas “no ions.” In a deionization process, water goes through anion-exchange and/or reverse osmosis process in order to remove ionsdissolved in the water (i.e. calcium, potassium, chlorine, fluorine,etc.) or other ionic impurities. This process may make the water purerand may control pH. In actuality, DI water still has ions because at alltemperatures above absolute zero, water thermally dissociates intohydroxide ions and hydrogen ions (protons). In another embodiment,non-DI water is used to discharge conductive interconnect line 32 andtungsten plug 30 connected thereto and the underlying conductive line 22connected to tungsten plug 30. In yet other embodiments, the water usedto discharge conductive interconnect line 32 and tungsten plug 30connected thereto and the underlying conductive line 22 connected totungsten plug 30 may be: degasified and deionized; deionized but notdegasified; degasified but not deionized; or neither degasified nordeionized.

FIGS. 9 and 10 graph the results of testing ICs during a 16 monthperiod. The tested ICs are identical in design and were made with andwithout the step of dipping the ICs into a DI water bath for 120 minutesor less prior to exposure to an alkaline cleaning solution to removepolymer residue. ICs tested after month 13 were made using a 120 minuteor less DI water-dip prior to polymer residue removal in accordance withone embodiment of the present invention, while ICs tested before month13 were not made with the process step of dipping into DI water for 120minutes or less prior to polymer residue removal. Except for the DIwater dip step, the ICs tested were made using identical manufacturingtools and processes.

FIG. 9 shows that ICs (dies) made with the DI water dip step on averagewere less prone to failure as a result of tungsten plug corrosion whencompared to ICs made without the DI water dip step. FIG. 10 shows thaton average, the process yield (ICs that functioned properly versus ICsthat failed to function properly) is higher when the DI water dip isused in the manufacturing process.

Although the present invention has been described in connection withseveral embodiments, the invention is not intended to be limited to thespecific forms set forth herein. On the contrary, it is intended tocover such alternatives, modifications, and equivalents as can bereasonably included within the scope of the invention as defined by theappended claims.

1. A method comprising: forming a tungsten plug in a dielectric layer;forming an electrically conductive interconnect line on the dielectriclayer after formation of the tungsten plug, wherein the tungsten plug iselectrically connected to the electrically conductive interconnect line;contacting the electrically conductive interconnect line with liquidwater after formation of the electrically conductive interconnect line;contacting the electrically conductive interconnect line with a solutionto remove residual polymer after the electrically conductiveinterconnect line is contacted with the liquid water; wherein theelectrically conductive interconnect line is contacted with the liquidwater for less than 120 minutes.
 2. The method of claim 1 wherein theliquid water is degasified and deionized.
 3. The method of claim 1wherein the liquid water is deionized but not degasified.
 4. The methodof claim 1 wherein the liquid water is degasified but not deionized. 5.The method of claim 1 wherein the liquid water is neither degasified nordeionized.
 6. The method of claim 1 wherein the liquid water has a pHthat is at or near neutral.
 7. The method of claim 1 wherein theelectrically conductive interconnect line is contacted with the liquidwater for less than 60 minutes.
 8. The method of claim 1 wherein theelectrically conductive interconnect line is contacted with the liquidwater for less than 15 minutes.
 9. The method of claim 1 wherein theelectrically conductive interconnect line is formed from a metal stackthat includes one or more of titanium, titanium nitride, aluminum, analuminum copper alloy, and an aluminum silicon copper alloy.
 10. Amethod comprising: forming a tungsten plug in a dielectric layer;forming an electrically conductive interconnect line on the dielectriclayer after formation of the tungsten plug, wherein the tungsten plug iselectrically connected to the electrically conductive interconnect line;contacting the electrically conductive interconnect line with liquidwater having a pH slightly less than neutral after formation of theelectrically conductive interconnect line; contacting the electricallyconductive interconnect line with a solution to remove residual polymerafter the electrically conductive interconnect line is contacted withthe liquid water; wherein the electrically conductive interconnect lineis contacted with the liquid water for less than 120 minutes.
 11. Themethod of claim 10 wherein the liquid water is degasified and deionized.12. The method of claim 10 wherein the liquid water is deionized but notdegasified.
 13. The method of claim 10 wherein the liquid water isdegasified but not deionized.
 14. The method of claim 10 wherein theliquid water is neither degasified nor deionized.
 15. A methodcomprising: forming a tungsten plug in a dielectric layer; forming anelectrically conductive interconnect line on the dielectric layer afterformation of the tungsten plug, wherein the tungsten plug iselectrically connected to the electrically conductive interconnect line;contacting the electrically conductive interconnect line with liquidwater after formation of the electrically conductive interconnect line;contacting the electrically conductive interconnect line with a solutionto remove residual polymer on the electrically conductive interconnectline after the electrically conductive interconnect line is contactedwith the liquid water; wherein the electrically conductive interconnectline is contacted with the liquid water for less than 120 minutes. 16.The method of claim 15 wherein the liquid water is degasified anddeionized.
 17. The method of claim 15 wherein the liquid water isdeionized but not degasified.
 18. The method of claim 15 wherein theliquid water is degasified but not deionized.
 19. The method of claim 15wherein the liquid water is neither degasified nor deionized.
 20. Themethod of claim 15 wherein the liquid water has a pH that is at or nearneutral.